Energy and industrial plants are required by law to capture certain contaminants emitted from plant operations. Some of these contaminants are generated by combustion in furnaces and/or boilers utilized to generate steam for various purposes. Flue gas, a natural by-product of combustion, carries a number of these contaminants, including sulfur. To remove sulfur from the flue gas, the flue gas is passed through a Wet Flue Gas Desulfurizer (WFGD) system before the flue gas exits the plant, typically through a chimney stack.
Environmental Protection Agency (EPA)—Mercury Air Toxics Standards (MATS) effective in 2017 create a new, more stringent regulation for Mercury and allows a sulfur dioxide (SO2) measurement as a proxy for achievement. The new standard for power plants fueled by lignite coal is 0.015 lbs. SO2/MM Btu of fuel. (Standards are different for different fuels.)
To meet the new emission standards, power plant efficiency improvements are frequently required. Many of these improvements have the effect of reducing flue gas temperatures at or in the chimney stack.
For chimney stacks designed to operate in a dry condition, the temperature of the flue gas in the chimney stack must be maintained above the flue gas water vapor dew point (“flue gas dew point”). If the temperature of the flue gas drops below the flue gas dew point, condensation of water vapor and acid gases ensues, which are very corrosive for these types of chimney stacks. Thus, reheat systems are required to keep the flue gas temperature above the dew point.
In the past, other reheat systems have been generally avoided in the industry due to high cost of operation and more significant maintenance requirements.
One solution to address this problem is to add a corrosive resistant heat exchanger in direct contact with the saturated flue gas to keep the flue gas warm, which is costly.
Another option is to redesign and replace the chimney stack so it operates in a wet condition, below the flue gas dew point, but this option is both capitally intensive and time consuming, limiting the magnitude and cost effectiveness of various emission reduction and efficiency improvements. A wet stack environment requires special design modifications and materials to avoid corrosion. These design modifications and the outage time to install would cost tens of millions of dollars in most applications.
Yet another option is to bypass a percentage of flue gas from each boiler around the flue gas desulfurization systems. This hotter flue gas mixes with flue gas desulfurization (FGD) outlet gas before the chimney stack and keeps the blended stack gas temperature well above the dew point temperature to keep chimney stack operating in a dry condition, above the flue gas dew point. (The FGD outlet gas is typically near the flue gas dew point as it exits the outlet.) Without this bypass gas to “reheat” the chimney stack gas, condensation will occur as the flue gas cools and rises through the chimney stack, leading to a corrosive “wet stack” environment. However, the cost for completing stack, ductwork, and drain modifications to successfully operate in a wet stack environment are extremely high and require long, costly operation outages to implement the modifications. Further, to meet the 2017 regulations, some power plants will need to scrub 100% of flue gas. This eliminates the opportunity to bypass a portion of the higher temperature flue gas and use its heat to raise the temperature of the lower temperature flue gas at the scrubber outlet. Without reheat, the fully saturated flue gas from the scrubber would condense in the stack, causing wet, acidic conditions leading to corrosion.
Thus, there remains a need for a low cost system to reheat the flue gas in a dry stack condition for energy and industrial plants looking to avoid the complications and retrofit expense of wet stack operations. Ideally, if sources of otherwise wasted heat can be utilized, the net plant heat rate (efficiency) of the power plant could be simultaneously improved.